In semiconductor device fabrication involving plasma processing to form nanometer-scale feature sizes across a large workpiece, a fundamental problem has been plasma uniformity. For example, the workpiece may be a 300 mm semiconductor wafer or a rectangular quartz mask (e.g., 152.4 mm by 152.4 mm), so that maintaining a uniform etch rate relative to nanometer-sized features across the entire area of a 300 mm diameter wafer (for example) is extremely difficult. The difficulty arises at least in part from the complexity of the process. A plasma-enhanced etch process typically involves simultaneous competing processes of deposition and etching. These processes are affected by the process gas composition, the chamber pressure, the plasma source power level (which primarily determines plasma ion density and dissociation), the plasma bias power level (which primarily determines ion bombardment energy at the workpiece surface), wafer temperature and the process gas flow pattern across the surface of the workpiece. The distribution of plasma ion density, which affects process uniformity and etch rate distribution, is itself affected by RF characteristics of the reactor chamber, such as the distribution of conductive elements, the distribution of reactances (particularly capacitances to ground) throughout the chamber, and the uniformity of gas flow to the vacuum pump. The latter poses a particular challenge because typically the vacuum pump is located at one particular location at the bottom of the pumping annulus, this location not being symmetrical relative to the either the workpiece or the chamber. All these elements involve asymmetries relative to the workpiece and the cylindrically symmetrical chamber, so that such key parameters as plasma ion distribution and/or etch rate distribution tend to be highly asymmetrical.
The problem with such asymmetries is that conventional control features for adjusting the distribution of plasma etch rate (or deposition rate) across the surface of the workpiece are capable of making adjustments or corrections that are symmetrical relative to the cylindrical chamber or the workpiece or the workpiece support. (Examples of such conventional features include independently driven radially inner and outer source-power driven coils, independently supplied radially inner and outer gas injection orifice arrays in the ceiling, and the like.) Such features are, typically, incapable of completely correcting for non-uniform distribution of plasma ion density or correcting for a non-uniform distribution of etch rate across the workpiece (for example). The reason is that in practical application, such non-uniformities are asymmetrical (non-symmetrical) relative to the workpiece or to the reactor chamber.
There is, therefore, a need to enable conventional control features for adjusting distribution of plasma process parameters (e.g., distribution across the workpiece of either etch rate, or etch microloading, or plasma ion density, or the like) to correct the type of asymmetrical or non-symmetrical non-uniformities that are encountered in actual plasma process environments.